The present invention pertains to an electrochemical oxygen concentrator for separating oxygen from a gas mixture containing oxygen.
A device with an oxygen concentrator for separating oxygen from ambient air has become known from DE 197 54 213 C1. The prior-art oxygen concentrator is based on the principle of ion transport in solids and has as its essential components a solid electrolyte with electrodes arranged thereon in opposite positions. The electrochemical cell thus formed is heated to a temperature between 700xc2x0 C. and 1,000xc2x0 C. and is connected to an ionization voltage source. The ionization voltage is on the order of magnitude of about 1 V.
If the gas mixture from which oxygen is to be separated is passed over the cathode, oxygen is ionized and is transported by the driving potential of the ionization voltage present on the electrochemical cell to the anode, where a deionization takes place and pure oxygen can be removed.
Zirconium dioxide, which is stabilized by doping in its cubic structure, is used as the electrolyte in the prior-art oxygen concentrator. The doping of zirconium dioxide, e.g., with calcium, yttrium or magnesium oxide generates oxygen ion vacancies, which are decisive for the oxygen ion conduction. The range of application of electrolytes based on zirconium dioxide in terms of the temperature is usually above 800xc2x0 C.
High requirements are also imposed on the electrode material of the prior-art electrochemical cell. Besides good electric conductivity, the electrodes must have a high porosity in order to allow the reaction gas, air in this case, to reach the interface between the electrode and the electrolyte. So-called perovskite materials are used as the electrode materials; the principal structure of these materials is AxByO3 (A and B=trivalent cations), and they have a sufficient conductivity for an air electrode.
An electrochemical cell in which the electrodes are arranged in a sandwich-like pattern at the electrolyte and the system of layers consisting of electrolyte and electrodes is stabilized by housing plates has been known from U.S. Pat. No. 5,868,918. The electrolyte in this electrochemical cell must have a certain minimum thickness of about one mm in order to still be able to receive the electrode material. In addition, additional sealing materials are necessary in the edge area between the housing plates and the electrolyte.
The primary object of the present invention is to provide an electrochemical cell which can be manufactured in a simple manner and at low cost and in which both the electrodes and the electrolyte can be prepared as thin layers.
An electrochemical cell is provided for separating oxygen from a gas mixture which contains oxygen as one of its components. The cell has a metallic housing plate and a porous gas-permeable metallic carrier plate located on the housing plate. A system of layers is provided on the carrier plate comprising a first electrode, an electrolyte, and a second electrode exposed to the gas mixture. At least one discharge opening is provided at the housing plate for drawing off the oxygen generated.
The advantage of the present invention is essentially that the system of layers consisting of electrodes and electrolyte is stabilized by a carrier plate permeable to oxygen, so that both the electrolyte and the electrodes can be prepared as thin, non-self-supporting layers. On the one hand, this leads to a low consumption of the very expensive electrode and electrolyte materials, and, on the other hand, the carrier plate, which is used for mechanical stabilization only, can be made of an inexpensive material. The carrier plate must be such that it is permeable to oxygen molecules. Suitable materials for the carrier plate are, e.g., metallic sintered materials.
Since both the electrodes and the electrolyte are not subject to great mechanical stresses by the subjacent carrier plate, brittle and fragile substances may be used for them, which have hitherto only been used with a greater material thickness. Due to the application of the electrodes and of the electrolyte layer by layer and the use of these substances, the working temperature of the electrochemical cell can be lowered to a value below 500xc2x0 C., which has a favorable effect on the energy consumption.
The housing plate may advantageously be provided with bead-like depressions. As a result, channels are formed between individual electrochemical cells stacked on each other, through which the gas mixture, from which oxygen is to be separated, is to flow. In addition, the bead-like depressions cause the housing plate to be stabilized against bending and torsional stresses.
The housing plate of the electrochemical cell may advantageously be designed such that it has a side wall surrounding the carrier plate in a pot-shaped manner and a collar which projects from the side wall and extends in parallel to the top side of the carrier plate. The height of the side wall is selected to be such that the top side of the carrier plate and the collar extend flush with one another. To connect the carrier plate to the housing plate in a conductive manner, the two parts are welded together or pressed to one another in the area of the side walls and the bead-like depressions. The first electrode, the electrolyte and the second electrode are then arranged in a layer structure on both the collar and the top side of the carrier plate. The topmost second electrode is exposed as a cathode to the gas mixture.
Sealing of the carrier plate against the environment can be advantageously achieved by means of a gas-tight wall section extending along the lateral surface of the carrier plate. This wall section can be prepared, e.g., by pressing together the carrier plate along the side walls. The pores of the carrier plate are sealed by the pressing together. An alternative possibility is to apply a gas-tight coating along the side walls. The sealing by means of a gas-tight wall section is suitable for housing plates which have no pot-shaped raised side walls.
The layer thickness of the first electrode, i.e., the anode, is preferably in the range of 50 xcexcm. The thickness of the electrolyte is preferably in the range of 10 xcexcm to 20 xcexcm. The second electrode, i.e., the cathode, has a preferred layer thickness of about 250 xcexcm. The greater layer thickness for the second electrode arises from the circumstance that a larger electrode surface is needed for the ionization of the oxygen than for the deionization at the first electrode. In addition, a uniform current distribution must be present within the second electrode, because the second electrode is contacted in a punctiform manner only by the superjacent housing plate of an adjacent electrochemical cell.
The electrolyte is preferably formed as an electrolyte overlapping the first electrode in the area of the collar. As a result, improved sealing is achieved in the area of the collar, because the oxygen is no longer able to escape laterally through the porous first electrode.
The electrolyte preferably consists of gadolinium-doped cerium oxide, Ce0.8Gb0.2O1.9 (CGO).
The electrode material is La perovskite, La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF).
Both the housing plate and the carrier plate are made of comparatively inexpensive materials, such as high-chromium steels with a chromium content greater than 20%. The housing plate preferably consists of a high-chrome, ferritic steel, and the carrier plate is preferably sintered together from high-chromium steel particles.
To generate an oxygen flow sufficient for practical applications, electrochemical cells are stacked on one another in the form of a layer structure. The contacting between adjacent cells is always between the underside of the housing plate of one cell and the second electrode of an adjacent cell. The ionization voltage source is connected to the series connection of the individual cells in the known manner. About 3 L of oxygen can be generated per minute with a layer structure consisting of 15 electrochemical cells and a gas exchange area of about 100 cm2 per cell.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.